Several dyes that absorb and emit light in the visible and near-infrared region of electromagnetic spectrum are currently being used for various biomedical applications due to their biocompatibility, high molar absorptivity, or high fluorescence quantum yields. The high sensitivity of the optical modality in conjunction with dyes as contrast agents parallels that of nuclear medicine and permits visualization of organs and tissues without the undesirable effect of ionizing radiation. Cyanine dyes with intense absorption and emission in the near-infrared (NIR) region are particularly useful because biological tissues are optically transparent in this region (B. C. Wilson, Optical properties of tissues. Encyclopedia of Human Biology, 1991, 5, 587-597). For example, indocyanine green, which absorbs and emits in the NIR region has been used for monitoring cardiac output, hepatic functions, and liver blood flow (Y-L. He, H. Tanigami, H. Ueyama, T. Mashimo, and I. Yoshiya, Measurement of blood volume using indocyanine green measured with pulse-spectrometry: Its reproducibility and reliability. Critical Care Medicine, 1998, 26(8), 1446-1451; J. Caesar, S. Shaldon, L. Chiandussi, et al., The use of Indocyanine green in the measurement of hepatic blood flow and as a test of hepatic function. Clin. Sci. 1961, 21, 43-57) and its functionalized derivatives have been used to conjugate biomolecules for diagnostic purposes (R. B. Mujumdar, L. A. Ernst, S. R. Mujumdar, et al., Cyanine dye labeling reagents: Sulfoindocyanine succinimidyl esters. Bioconjugate Chemistry, 1993, 4(2), 105-111; Linda G. Lee and Sam L. Woo. "N-Heteroaromatic ion and iminium ion substituted cyanine dyes for use as fluorescent labels", U.S. Pat. No. 5,453,505; Eric Hohenschuh, et al. "Light imaging contrast agents", WO 98/48846; Jonathan Turner, et al. "Optical diagnostic agents for the diagnosis of neurodegenerative diseases by means of near infra-red radiation", WO 98/22146; Kai Licha, et al. "In-vivo diagnostic process by near infrared radiation", WO 96/17628; Robert A. Snow, et al., Compounds, WO 98/48838).
A major drawback in the use of cyanine dye derivatives is the potential for hepatobilliary toxicity resulting from the rapid clearance of these dyes by the liver (G. R. Cherrick, S. W. Stein, C. M. Leevy, et al., Indocyanine green: Observations on its physical properties, plasma decay, and hepatic extraction. J. Clinical Investigation, 1960, 39, 592-600). This is associated with the tendency of cyanine dyes to form aggregates in solution which could be taken up by Kupffer cells in the liver. Various attempts to obviate this problem have not been very successful. Typically, hydrophilic peptides, polyethyleneglycol or oligosaccharide conjugates have been used but these resulted in long-circulating products which are eventually cleared by the liver. Another major difficulty with current cyanine and indocyanine dye systems is that they offer a limited scope in the ability to induce large changes in the absorption and emission properties of these dyes. Attempts have been made to incorporate various heteroatoms and cyclic moieties into the polyene chain of these dyes (L. Strekowski, M. Lipowska, and G. Patonay, Substitution reactions of a nucleofugal group in hetamethine cyanine dyes. J. Org. Chem., 1992, 57, 4578-4580; N. Narayanan, and G. Patonay, A new method for the synthesis of heptamethine cyanine dyes: Synthesis of new near infrared fluorescent labels. J. Org. Chem., 1995, 60, 2391-2395; E. Fung and R. Rajagopalan, Monocyclic functional dyes for contrast enhancement in optical imaging, U.S. Pat. No. 5,732,104; R. Rajagopalan and E. Fung, Delta.sup.1,6 bicyclo[4,4,0] functional dyes for contrast enhancement in optical imaging, U.S. Pat. No. 5,672,333; R. Rajagopalan and E. Fung, Tricyclic functional dyes for contrast enhancement in optical imaging, U.S. Pat. No. 5,709,845) but the resulting dye systems do not show large differences in absorption and emission maxima, especially beyond 830 nm where photacoustic diagnostic applications are very sensitive. They also possess prominent hydrophobic core which enhances liver uptake. Further, most cyanine dyes do not have the capacity to form dendrimers which are useful in biomedical applications.
Therefore, there is a need to design novel dyes that could prevent dye aggregation in solution, predisposed to form dendrimers, capable of absorbing or emitting beyond 800 nm, possess desirable photophysical properties, and endowed with tissue-specific targeting capability.
The publications and other materials used herein to support the background of the invention or provide additional details respecting the practice, are incorporated by reference.